The function of a demand value is to feed compressed air to the mask wearer at the proper pressure and flow to exactly reflect his respiratory needs. During exhale the valve is closed and no air flows from the air source to conserve compressed air supplies.
Demand valves have been in general use for many years in SCUBA Diving Equipment, SCBA--Self Contained Breathing Apparatus for Fire Fighting, Aviators Oxygen Breathing Systems and so forth. The usual design consists of an air valve with the actuating mechanism connected to a flexible diaphragm that reacts to small pressure changes within the face mask. When the person wearing the mask inhales, the pressure within the face piece is reduced below the ambient outside pressure, this differential pressure is sensed by the demand valve and the valve opens sufficiently to bring the pressure within the face piece back to ambient pressure and shuts off air flow. With the prior art valve systems some physical inhaling effort is required by the mask wearer to obtain air. Under hard work conditions this extra effort can become very tiring and for this reason there have been many valve designs that have attempted to reduce the amount of effort required to actuate the valve. Two different methods are generally employed to reduce the effort required to actuate the valve.
One method is to reduce to a minimum the amount of force required to actuate the valve by designing into the valve several stages of pressure control and by balancing the air forces with spring mechanisms. This method works well but requires many precision components, careful set up, adjustments, and repeated maintenance adjustments to keep it operating correctly.
The other method is to let the flow of air through the valve body act as a boot strapping effect to assist in opening the valve. This has been employed in the past by deflecting all or part of the air entering the valve towards the outlet opening which by venturi effect helps to draw in the diaphragm which in turn decreases the effort required from the mask wearer.